AND LEICESTERSHIEE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 91 
“ The effects at C and D are perhaps not so clearly related to the southward 
extension of the iron-ore sheet. The outcrop follows, however, a rather irregular 
course on the "south side of the valley, and this may introduce complications in the 
local disturbances. Further, it must be kept in mind that, owing to experimental 
error and uncertainty as to the datum-plane, there might be superposed a force of a 
magnitude sufficient to reduce C to zero and, as it happens, the same force would also 
be sufficient to reduce D to an extremely small horizontal disturbance and a slightly 
greater upward disturbance.”—G.W,W. 
Fig. 4. Section to illustrate distribution of lines of force arising from a horizontal sheet magnetised 
by earth’s induction. 
“ On the other hand, even if a force that would reduce C to zero were superposed 
on the forces as determined, the effects at the points A, B, and E would not be modi¬ 
fied in essential features. At A the horizontal disturbance to the south would be in¬ 
creased, while the downward vertical disturbance would be reduced. At B the 
small -T vertical disturbance would be converted into a — vertical disturbance, but 
the big horizontal disturbance, the characteristic feature at B, would still remain 
and its value would be slightly increased. At E the horizontal disturbance would 
be reduced, while the — vertical disturbance, the characteristic feature would be 
increased.”—G.W.W. 
“ Thus the observations at three of the stations—A, B, and E—agree with the 
results to be expected from the geological structure, while the results at two 
stations—C and D —are inconclusive, perhaps owing to the irregular nature of the 
outcrop in their vicinity. Applying the rough guide from the susceptibilities of 
the rocks (see p. 82 ), there is harmony between the observed forces, the known 
geology and the measured susceptibility of the rocks. It thus appears highly 
probable that the known ironstone is the vera causa of the observed forces.”— 
G.W.W. 
